The formula behind every reconstitution chart
Concentration (mg per mL) equals the vial’s total mg divided by the water volume in mL added. Every dosing chart, calculator or worked example ultimately comes back to this one relationship.
Working backwards from a target concentration
Sometimes it’s easier to start from a concentration you want (say, a round number like 5mg/mL) and solve for the water volume: water volume equals vial mg divided by target concentration. For a 10mg vial targeting 5mg/mL, that’s 10 ÷ 5 = 2mL of water.
From concentration to a syringe draw
Once concentration is known, units for a given dose equal the target mg divided by the concentration, multiplied by 100 (since a standard insulin syringe reads 100 units per mL). This is the same formula used across every reconstitution calculator, whether for retatrutide, tirzepatide, or any other peptide sold as a lyophilised vial.
Why choosing round numbers helps
A reconstitution volume that produces whole-number unit draws for your typical doses is easier to measure accurately and easier to sanity-check at a glance than one that lands on an awkward fraction.
Double-checking your own maths
Recalculating from the vial’s actual label, rather than reusing a remembered number, is worth doing every time a new vial is opened, since a mismatched water volume assumption is the most common source of dosing errors.